Why are there rules about notching and drilling joists?

Notches and holes weaken joists by removing material from the cross-section. Notches in the wrong position (mid-span) can cause the joist to split or fail in shear. Holes that are too large or too close together reduce the effective depth and bending strength. The rules in Approved Document A (referencing BS 8103-3) define safe zones and maximum sizes to ensure the joist retains adequate structural capacity.

What if I need a notch or hole outside the permitted zones?

If you need to cut or drill a joist outside the permitted zones, or exceed the maximum sizes, a structural engineer must assess the specific situation. Options include using joist reinforcement plates (galvanised steel plates bolted either side of the notch), using deeper joists to compensate, or rerouting the pipe/cable to avoid the conflict. Never notch or drill engineered joists (I-joists, glulam) without following the manufacturer's specific guidelines.

Do these rules apply to engineered timber joists?

No. Engineered timber joists (I-joists, metal web joists, glulam beams) have their own manufacturer-specific rules for service holes. I-joists typically have designated knockout holes in the web and prohibit notching or cutting the flanges. Metal web joists allow services to pass between the webs without any cutting. Always follow the manufacturer's installation guide for engineered products.

Can I notch the bottom of a joist?

Notches should only be cut in the top edge of a joist. The bottom edge is in tension under load and cutting a notch there would create a stress concentration that could cause the joist to crack and fail. All notch rules assume top-edge notching only. If a pipe must pass below the joist, use a deeper joist or route the pipe along the joist rather than through it.

Notching and Drilling Rules for Joists

Notching Rules (Top Edge Only)

Reference: Approved Document A, Diagram 7 / BS 8103-3. D = joist depth, L = span.

Rule	Requirement	Example (200mm joist, 4m span)
Maximum notch depth	D / 8 (one-eighth of joist depth)	200 / 8 = 25mm max
Permitted zone (from support)	0.07L to 0.25L from face of support	280mm to 1000mm from wall face
Notch location	Top edge only — never bottom edge	—
At bearings	Notches at the bearing point are permitted	e.g., notch to clear wall plate

Drilling Rules (Holes Through Joist)

Rule	Requirement	Example (200mm joist, 4m span)
Maximum hole diameter	D / 4 (one-quarter of joist depth)	200 / 4 = 50mm max
Permitted zone	0.25L to 0.4L from face of support	1000mm to 1600mm from wall face
Hole position (vertical)	On the neutral axis (centre of depth)	Centre of 200mm = 100mm from top/bottom
Minimum spacing between holes	3 x diameter of largest hole (centre to centre)	3 x 50 = 150mm min between holes
Distance from notch	Not in the same cross-section as a notch	—

Quick Reference — Common Joist Sizes

Joist Depth (mm)	Max Notch (D/8)	Max Hole (D/4)
150	18.7mm	37.5mm
175	21.9mm	43.8mm
200	25.0mm	50.0mm
225	28.1mm	56.3mm

Common Pipe Sizes vs Max Hole Diameters

Service	Pipe OD (mm)	Hole Needed (mm)	Min Joist Depth (mm)
15mm copper pipe	15	20	80
22mm copper pipe	22	28	112
28mm copper pipe	28	35	140
40mm waste pipe	40	45	180
110mm soil pipe	110	—	Cannot drill — use joist hangers or notch at bearing

Note: Always refer to the current edition of Approved Document A and BS 8103-3 for definitive requirements. These rules apply to solid sawn timber joists only — engineered timber products have their own manufacturer-specific rules.

See the Joist Span Tables, the UK Timber Sizes reference, and the UK Pipe Sizes.

Notching and Drilling Rules for Joists

Notching Rules (Top Edge Only)

Drilling Rules (Holes Through Joist)

Quick Reference — Common Joist Sizes

Common Pipe Sizes vs Max Hole Diameters

Related Pages

How We Calculate This

Frequently Asked Questions

Related Calculators

Concrete Mix Ratios

Insulation U-Values

UK Brick Dimensions

UK Cable Sizes